This commit switches the tox command to use ostestr instead of calling
testr through setup.py. The primary advantage here is that it uses the
subunit-trace output filter. ostestr primarily exists as a replacement
for pretty_tox.sh bash scripts which spread like a plague through
OpenStack projects after Nova copied it from Tempest. (although
ostestr also provides some other useful features) Granted ironic-specs
wasn't using that, but this commit makes the switch to enable getting
the subunit-trace output which is generally useful.
Note that this dramatically increases output while running tests.
However, test failures are still at the bottom of the output, so it
shouldn't cause much pain, if any.